Although virtually all of the structural steel from the Twin Towers and Building 7 was removed and destroyed, preventing forensic analysis, FEMA's volunteer investigators did manage to perform "limited metallurgical examination" of some of the steel before it was recycled. Their observations, including numerous micrographs, are recorded in Appendix C of the WTC Building Performance Study. Prior to the release of FEMA's report, a fire protection engineer and two science professors published a brief report in JOM disclosing some of this evidence. 1

The results of the examination are striking. They reveal a phenomenon never before observed in building fires: eutectic reactions, which caused "intergranular melting capable of turning a solid steel girder into Swiss cheese." The New York Times described this as "perhaps the deepest mystery uncovered in the investigation." 2 WPI provides a graphic summary of the phenomenon.

A one-inch column has been reduced to half-inch thickness. Its edges--which are curled like a paper scroll--have been thinned to almost razor sharpness. Gaping holes--some larger than a silver dollar--let light shine through a formerly solid steel flange. This Swiss cheese appearance shocked all of the fire-wise professors, who expected to see distortion and bending--but not holes.
FEMA's investigators inferred that a "liquid eutectic mixture containing primarily iron, oxygen, and sulfur" formed during a "hot corrosion attack on the steel." The eutectic mixture (having the elements in such proportion as to have the lowest possible melting point) penetrated the steel down grain boundaries, making it "susceptible to erosion." Following are excerpts from Appendix C, Limited Metallurgical Examination.

Evidence of a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent intergranular melting, was readily visible in the near-surface microstructure. A liquid eutectic mixture containing primarily iron, oxygen, and sulfur formed during this hot corrosion attack on the steel.
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The thinning of the steel occurred by high temperature corrosion due to a combination of oxidation and sulfidation.
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The unusual thinning of the member is most likely due to an attack of the steel by grain boundary penetration of sulfur forming sulfides that contain both iron and copper.
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liquid eutectic mixture containing primarily iron, oxygen, and sulfur formed during this hot corrosion attack on the steel.
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The severe corrosion and subsequent erosion of Samples 1 and 2 are a very unusual event. No clear explanation for the source of the sulfur has been identified. The rate of corrosion is also unknown. It is possible that this is the result of long-term heating in the ground following the collapse of the buildings. It is also possible that the phenomenon started prior to collapse and accelerated the weakening of the steel structure. A detailed study into the mechanisms of this phenomenon is needed to determine what risk, if any, is presented to existing steel structures exposed to severe and long-burning fires.

Thermite Use as an Explanation
The "deep mystery" of the melted steel may be yielding its secrets to investigators not beholden to the federal government. Professor Steven Jones has pointed out that the severe corrosion, intergranular melting, and abundance of sulfur are consistent with the theory of thermite arson.